Power line carrier frequency telephone system



Dec. 2, 1941. H. R. MOORE 2,264,396

POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM Filed Oct. 22', 1940 fi 30 FIG.

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D lM/EA/TOR H. R. MOO/PE By g" f A? ATTOR v Patented Dec. 2, 1941 POWER LINE CARRIER FREQUENCY TELEPHONE SYSTEM Hilbert R. Moore,. Phickemim N. J'., assignor to Bell Telephone Laboratories, Incorporated,- New York, N Y., acorporation of'N'ew York Application ct'oher'22, 1940, SerialNo. 362,244

(CL- 17-fi2.5)

7- Claims.

systems, however, the carrier frequency source or' sources have been provided at each telephone station coupled to the power line, as well as at any central office or' exchange for the system.

An object of this invention is to provide a', power line telephone system in which a single carrier frequency source onlyisrequired;

When a carrier wave source common to the system is employed there is thepossibility thatthere will be phase differences between thecarrier wave at the calling station and at the called station.

Another object of the invention, therefore; is to minimize any adverse eifect's on telephonic communication that might result from such phase differences or phase shift.

A power line carrier frequency telephone system in accordance with the invention comprises a transmission line for transmitting electric power, for example, of high or low voltage and of relatively low frequency, between two or more points and to a plurality of power consumers some or all of whom, together with others in the locality through which the line extends, desire telephone service. Telephone subscribers" stations are coupled to the power line and are adapted to receive and to transmit telephone signals on high frequency or carrier waves; At each station and at the central office-for the system, means are provided for modulating the carrier waves for transmission outgoing from the station or ofiice, and for demodul'ating a modulated carrier wave incoming to the station or office. A single source of carrier' wave common to the system is located; at the central office, or at one of the stations, or may be coupled to the power line at some point along the line outside of a station or the central oflice: Because the distances between stations and between a station and the central ofiice may be appreciable and will vary with the stations involved in a particular call, there may be phase differences between the carrier wave at the called or calling station and that of the calling or called station, that is, there may be phase differences in transmission occurring between the carrier wave employedfor demodulation. Any adverse effect, i. e., loss in demodulation output at the receiving station, that might follow from such a condition may be precluded by arranging the stations and the central otliceto transmit and to receive onlythe carrier wave and one side-band of the modulated carrier wave. In the event that it should be desired tic-transmit and to receive the carrier wave and both side-bands of the modulated carrier wave, each station and a central office (if the carrier" wave source is not located thereat) are arranged to include phase correcting means.

A more complete understanding of this invention will be obtained from the detailed description which follows taken in conjunction with the appended drawing wherein:

1 Fig; 1 shows a power line carrier frequency telephone system incorporating the invention;

Fig. 1A shows an alternative location for the singlesource of carrier wave for the system of Fig. l; and

2 shows a circuit arrangement for inclusionin amodification of the circuit of Fig. 1.

The system shown in Fig. l is illustrative of the invention and comprises a plurality oftelephone subscribers stations A and B, coupled to a power line PL or other'transmission media and a central office or exchange for the stations A, B; and such other stations as are coupled to the power line. As each telephone station will be the same, station A only is shown: in detail.

The station A comprises a microphone or transmitter lll which may be of the carbon granule type; atelephone receiver #2; an induction coil-i If, one winding of which is adapted: to be connected in series with the transmitter and a local battery l6; when the receiver has been removed from the switch hook l8, and another winding of which is adapted to be connected in serieswith the receiver across one pair of terminals a, w of amodulato1-'-demodulator 20'. It will be-not ecl'l that'reg-ardlessof the position of the switch-hook the receiver circuit remains closed; normally, that is, when the subscriber at station A is-not using his equipment, the receiver will be supported on the switchhook, and the battery circuit-for the transmitter will be opened at the upper pair of. switchhook contacts. Also: connected across the modulator demod-ulator terminals a; 11,. is. a station signaling or ringing circuit lfcomprisinga ringer 2-4 adapted to respond, for example, to a. ZO-cycl'e signal, which; maybe preceded by an amplifier 2.6,v thev ringing. circuit normally being closed through the lower pair of modulation and the carrier wave employed for switchhook contacts. A series circuit of. a normally-open signaling key 28 and a circuit interrupter 30 adapted to interrupt the circuit, for example, twenty times a second when current is supplied to it, and to generate a 20-cycle per second signal in the receiver winding of the induction coil, is connected in parallel with the transmitter.

The modulator-demodulator may be of any suitable type, but it is preferred to employ a bridge arrangement of non-linear current-resistance characteristic devices, for example, copper oxide rectifier units. A wave frequency selective network or filter 32 is connected to the terminals b, b of the modulator-demodulator and to one winding of a transformer 34 provided to match the impedance of the subscribers station to that of the power line. Of course, any other fixed or adjustable impedance matching device may be employed for the latter purpose. In accordance with one feature of the invention, the filter is designed to pass the carrier wave and either the upper or the lower side-band of the modulated carrier wave. That is, if the carrier wave frequency is denominated f and when modulated its side-bands are fifi, where f1 would be the upper frequency, for example, 3500 cycles per second, of the voice frequency band to be transmitted and received at the stations in the system, the filter passes the band I through f+fi or the band 1 through f-f1. The power line winding of transformer 34 is coupled to the power line through a condenser 36, a suitable protector 38 being connected across the winding on the station side of the coupling condenser to preclude hazard to the subscriber or his equipment from high voltages arising in the power line. In the case of a single phase grounded neutral power line such as is shown only one coupling condenser is needed; with a fully insulated single phase power line, a coupling condenser in each line connection should be provided.

The central office circuit comprises a coupling condenser 36, protector 38, impedance matching transformer 34, a band-pass filter 32 and a modulater-demodulator 20, as in the case of a subscribers station. A source 40, however, of carrier wave, for example, any suitable constant frequency oscillator, is connected across the terminals b, b of the modulator-demodulator, and an operator's signal lamp circuit 42 is connected across its terminals a, a, the signal lamp circuit being completed through the-operators line jack 44 coupled through a transformer 46 to the terminals a, a of the modulator-demodulator. The operator at the central office would be provided, of course, with a suitable operator's telephone set (not shown) for talking and listening through the line jack. The signal lamp circuit comprises a relay 48 preceded by an amplifier 50 for oper-- ating at a low audio frequency, for example, 20 cycles per second, to close the signal lamp circuit when signaling current at that frequency is incoming to the central office from one of the subscribers stations. Insertion of the operators telephone set plug in the jack 46 opens the signal lamp circuit.

The operation of the system described is as follows: When a subscriber, for example, at station A wishes to call either the central oflice or another subscriber on the power line, he removes the receiver from the switchhook. This closes a circuit for the transmitter through the battery and induction coil winding, and connects battery to the interrupter. The calling party then closes and opens the interrupter circuit by use of the signaling key, the number of times required by the code employed in the system for calling a particular other subscriber or the central office. The low frequency alternating current (20 cycles per second) generated in the receiver winding of the induction coil is impressed on the modulator-demodulator acting as a modulator, and to which the carrier wave is continuously supplied from the carrier wave source at the central office. Among the products of modulation will be frequencies equal to the sum and difference of the carrier and the signal frequency. The carrier and the upper or lower side-band is passed by the filter to the power line.

At the called partys station, the incoming high frequency wave is demodulated by the modulatordemodulator acting as demodulator. If it is assumed that the called partys receiver is on its switchhook, the resulting low frequency signal is applied to the ringer through the ringing circuit amplifier. At the central office the low frequency relay is operated instead of a ringer, and closes the local battery circuit for the signal lamp. After the called party answers, or the operator respondsif she has been signaled, telephonic communication takes place, with the voice frequency currents modulated on the carrier wave at the talking station and the modulated carrier being demodulated at the listening station.

When the operator wishes to call a station on the power line, a low frequency (20 cycles per second) ringing voltage is applied throughv the line jack to the central ofiice modulator-demodulator, known ringing arrangemcnts associated with an operators cord circuit being employed. The central office would be used primarily to connect a power line telephone system subscriber with a telephone subscriber in a general telephone system, an ordinary voice frequency transmission line being used to connect the power line system central office With that of the general telephone system.

In the system described, the source of carrier wave has been located at the central oihce. It may be located, however, at any convenient point in the system. For example, it could be coupled to the power line as indicated at AL in Fig. 1A, and be enclosed in a suitable pole-mounted or buried housing adjacent the power line. At both the central office and the subscribers station, furthermore, the carrier frequency equipment could be mounted in a suitable housing adjacent the power line, connection from the carrier equipment to the station and central oflice voice frequency and signaling circuits being over voice frequency transmission lines.

It may be found desirable to transmit and to receive both side-bands of the modulated carrier. When this is the case, however, phase correcting means 10 (Fig. 2) is required at each subscribers station (and at the central ofiice if the carrier wave source is located at AL, and the filter 32 is-replaced by a filter 32' designed to pass the band. f to fifi. The arrangement of Fig. 2 would replace filter 32 between CDCD' at each station, and a filter 32 would replace the filter 32 at the central oflice. The phase corrector Hi adjusts for the phase differences in transmission that may occur between the carrier wave employed for modulation and the carrier wave employed for demodulation. Without phase correction, and depending on the length of transmission paths and the frequency of the carrier wave, the average of the phases of the two side-bands may sometimes differ by an odd multiple of 90 degrees from the demodulation carrier wave, with the result that the two useful demodulation products would cancel. Only when the phase difference is zero degrees, or an even multiple of 90 degrees, would the full output be realized. On a party line system Where communication is desired with other parties as well as with the central office, the phase corrector should be adjustable to correct to the different line lengths that may be involved in different calls. The adjustment might be in any desired steps to produce the necessary phase shifts.

What is claimed is:

1. A power line carrier frequency telephone system comprising a power line, carrier frequency telephone stations coupled to said line, means at each station for modulating a carrier wave, for demodulating a modulated carrier wave, and for selectively transmitting the carrier wave and its side-bands, a common source of carrier wave for all of said stations, and means at each station to correct for difference in phase between the carrier wave at the station where modulated and at the station Where the modulated carrier wave is demodulated.

2. A power line carrier frequency telephone system comprising a power line, carrier frequency telephone stations coupled to said line, a central office for said stations coupled to said power line, means at each station and at said central ofiice for modulating a carrier wave, for demodulating a modulated carrier wave, and for selectively transmitting the carrier wave and its side-bands, a common source of carrier wave for all of said stations and said central oflice, said carrier wave source being coupled to the modulating and demodulating means at the central oflice, and means at each of said stations only, to correct for difference in phase between the carrier wave at the station where modulated and at the station where the modulated carrier wave is demodulated.

3-. A power line carrier frequency telephone system comprising a power line, carrier frequency telephone stations coupled to said line and adapted to communicate directly with each other over said power line on a single carrier wave for transmitting and receiving, said carrier wave being supplied to said stations from a common source located along said power line, and means at each station for modulating said carrier wave with voice and signaling frequency waves, for demodulating the modulated carrier wave, and for selectively transmitting and receiving said carrier and one side-band only of the modulated carrier wave.

4. A power line carrier frequency telephone system comprising a power line, carrier frequency telephone stations coupled to said line, a carrier frequency central oflice for said stations, said stations being adapted to communicate directly with one another and with the central oflice on a single carrier wave for transmitting and receiving, and a common source of carrier wave for said stations and said central office, each of said stations and said central office including means to transmit to and receive from said power line only the carrier wave and one side-band of the modulated carrier wave.

5. A power line carrier frequency telephone system comprising a power line, carrier frequency telephone stations coupled to said line, a carrier frequency central office for said stations, said stations being adapted to communicate directly with one another and with the central ofiice on the same carrier wave for transmitting and receiving, and a common source of said carrier wave for said stations and said central ofiice, each of said stations and central office including means to transmit and receive from said power line only a preassigned band of frequencies.

6. A power line carrier frequency telephone system comprising a power line, a plurality of carrier frequency telephone stations coupled to said line, a carrier frequency central office for said stations, each station and said central office including a combined modulator-demodulator having a pair of audio frequency terminals and a pair of carrier frequency terminals, an audio frequency receiving and transmitting circuit connected to said audio frequency terminals, a carrier frequency circuit connected between said carrier frequency terminals and said power line, said stations being adapted to communicate directly with one another and with the central office on the same carrier wave in transmitting from and receiving at said stations and central ofiice, and a source of said carrier wave for connection in the carrier frequency circuit for continuously supplying the carrier wave to the stations and the central office of the system, each of said stations and central office including means in the carrier frequency circuit thereof to transmit to and to receive from said power line only a preassigned band of frequencies but including said carrier wave.

7. A power line carrier frequency telephone system comprising a power line, a plurality of telephone stations and a central office for said stations coupled to said line, a single carrier frequency source for continuously supplying a single carrier frequency electric wave to said line, stations and central office, all carrier frequency transmitting from and receiving at said stations and central ofiice being on the same carrier frequency wave, and means at each station for directly signaling to and for carrying on a conversation with another station on the line and for signaling to and communicating with the central office, said means including a combined modulator-demodulator comprising a plurality of non-linear current-resistance characteristic devices, and a carrier frequency filter for transmitting only a preassigned band of frequencies including the carrier frequency wave to and from said combined modulator-demodulator.

HILBERT R. MOORE. 

